The Nutritional Properties of Soya Beans

Although a relative newcomer in the West, the soya bean has been cultivated for thousands of years in the Far East. Indeed, such was its importance that the ancient Chinese considered the soya bean to be one of the five sacred grains (along with rice, wheat, barley and millet). Recorded history traces it back as far as 2838 BC in China, to the 6th century in Japan, and to the 17th century in Europe. Today, the USA produces one-third of the world's supply, mostly for use in livestock feed and non-food products with only 2% destined for human consumption. However, in some areas of China, where soya foods are consumed in at least ten different forms, soya bean consumption may be as high as 55-60 grams per day. In the UK, the best-known derivatives of this versatile legume are tofu, soya milk and other soya 'dairy' products, and meat analogues such as Textured Vegetable Protein (TVP). Its widespread use in the food industry (it has been estimated that soya is used in 60% of processed foods) undoubtedly helps to explain why soya beans have become one of the first crops to be genetically modified - a source of considerable and justifiable concern to many consumers.
Like most legumes, soya beans are an excellent source of dietary fibre, complex carbohydrate and plant protein. On the negative side soya beans are relatively high in fat, although this is mainly unsaturated fat. For example, 100g dried, raw soya beans contain 35.9g protein, 15.8g carbohydrate, 8.5g water, and 18.6g fat, of which just 2.3g is saturated fat. Cooking naturally increases the water content whilst diluting the other nutrients, but 100g steamed tofu, for example, still contains 8.1g protein.
However, it is the micronutrient content of soya beans (and, to some extent, other legumes) which may be of greater significance. For example, phytochemicals such as saponins (found in a variety of legumes) and isoflavonic phytoestrogens ('plant estrogens' - found primarily in soya beans) may promote bone health, and reduce the risk of both cardiovascular disease and hormone-related cancers. Indeed, a recent study by the Cancer Research Center of Hawaii showed that women who ate foods rich in phytoestrogens had a reduced risk of endometrial cancer compared to women who consumed little on none of these foods. However, a review of 58 epidemiological studies reporting results for legume consumption and cancer risk by the World Cancer Research Fund showed inconsistent results, so that the jury is still out on this question.
More certain is the cholesterol-lowering effect of soya protein. A recent meta-analysis of the effects of soya protein on serum lipids in 29 controlled clinical trials showed that total cholesterol decreased by 9.3%, LDL-cholesterol ('bad' cholesterol) decreased by 12.9%, triglycerides decreased by 10.5%, and HDL-cholesterol ('good' cholesterol) increased by 2.4% in the intervention (soya protein) groups compared with the control groups. These beneficial effects on serum lipids are likely to result in a considerably reduced risk of coronary heart disease. Other potential benefits of legumes include their extremely low glycaemic index, which suggests that they may be a particularly important food for diabetics and individuals at risk of becoming insulin resistant, and the relief of menopausal symptoms.
Paul Appleby